1. Field of the Invention
The present invention is directed to antennas for receiving satellite signals. It finds particular application in antennas for use on mobile platforms.
2. Background Information
A typical mobile satellite antenna has a stationary base mounted to its platform, such as a boat or recreational vehicle, and a satellite-following rotatable assembly is mounted on the base for two- or three-axis rotation with respect to the base. That rotatable assembly includes a primary reflector, a secondary shaped sub-reflector, and a low-noise block down-converter, and it may also include gyroscopes for providing sensor inputs to the rotatable assembly's orientation-control system.
The reflectors are often arranged in Cassegrain configuration: microwaves from a small solid angle of sky are reflected by the paraboloidal primary reflector onto the smaller sub-reflector disposed in front the primary reflector. From the sub-reflector those microwaves are directed through a central opening in the primary reflector to the low-noise block down-converter. In some antennas, the sub-reflector focuses the radiation not to a point behind the primary reflector's central opening but rather to the mouth of a waveguide, or “feed tube,” disposed in front of the primary reflector to guide radiation through the primary-reflector opening to the low-noise block down-converter. The low-noise block down-converter down-converts a block of microwave television or other communications channels to an intermediate-frequency range, at which the channel signals propagate by cable off the rotating assembly to an IF strip mounted on the stationary base.
A typical mounting arrangement for this configuration includes a motor-driven turntable journaled in the base for rotation about one axis with respect to the base. Bearings on the turntable in turn journal the primary reflector for rotation with respect to an axis in the turntable's frame of reference, and a second servomotor cooperates with the turntable servomotor to keep the primary reflector aimed at the desired satellite.
The other rotatable-assembly elements are mounted in turn on the primary reflector. Conventionally, this is accomplished by providing a mounting bracket for the gyroscopes, which are disposed behind the primary reflector, and employing bolts to secure the low-noise block down-converter to the feed tube in such a manner as to sandwich the primary reflector and gyro bracket between them. The sub-reflector is in turn mounted on the feed tube, from which it is spaced by a reflector-mounting tube that is made of microwave-transparent material so that the feed tube does not block the path of microwaves traveling from the primary reflector to the sub-reflector.